1. Field of the Invention
The present invention relates to a circuit board and a method of mounting an integrated circuit chip, as a chip-on-board (COB).
2. Description of the Related Art
An integrated circuit chip (hereinafter called LSI) can be mounted on a wiring board as a flip chip and a connection thereto sealed with resin. Generally, the coefficient of thermal expansion of the wiring board is large, compared with that of the LSI. However, when a soft printed wiring board is used, the portion where the LSI is mounted is easily deformed on the side where LSI is mounted, because a difference in coefficients of thermal expansion between the wiring board and LSI causes a convex warp.
Referring to FIG. 15, a COB 100 may include active elements such as LSI 101 and a driver IC 112, a passive element such as a tantalum capacitor 112a, a chip capacitor 112b and a connector 112c mounted on wiring board 102. Sealing resin 103 is injected between LSI 101 and the wiring board 102, and a heat sink 110 for outgoing radiation is attached on LSI 101 via a thermal conduction member. The chip capacitor 112b that functions as a bypass capacitor of each LSI 101 is also soldered on the wiring board 102. Each terminal of the connector 112c is connected to LSI 101 and others via wiring provided on the surface or inside the wiring board 102. Therefore, COB 100 is electrically connected to an external device via the connector 112c. 
Referring to FIG. 16, when the heat sink 110 is attached to the back of LSI 101, the thickness of a thermal conduction member 109 is not uniform. When the distance between the heat sink 110 and LSI 101 increases in the periphery, the heat resistance increases, degrading the outgoing radiation. Also, depending upon a temperature change while LSI 101 is operated, the warp of the LSI may change, causing peeling of the thermal conduction member 109. When the warp is further increased, the LSI may crack.
To solve the problems described above, a structure in which the warp of LSI seldom changes is required.
Japanese published unexamined patent application No. Hei 10-229102 discloses a technique for preventing an LSI chip from being warped by mounting another LSI chip on the back of a wiring board opposite to the LSI chip mounted on the wiring board is. However, this technique can be applied only when two LSI chips having the same shape are packaged as one pair.
Also, a few hundred to more than a thousand solder bumps are generally provided on one side of an LSI chip. If an LSI chip is mounted on both sides of a wiring board, then an electrode pad is also required to be provided on both sides of the wiring board. Therefore, the structure of the wiring board becomes very complex and the manufacture is difficult. In the future, as large scale integrated circuits are enlarged, the number of the pins will increase. From the viewpoint of manufacturing a wiring board, it is not good policy to adopt the technique disclosed in the above-noted patent application to prevent the LSI from being warped.
The invention is made to solve the above-described problems and the object is to provide the circuit board and the method of mounting an integrated circuit chip wherein the warp of the LSI can easily be reduced when plural LSIs are mounted on a wiring board.
To achieve such an object, the circuit board according to the invention is based upon the structure of an integrated circuit chip provided with a wiring board and a plurality of integrated circuit chips mounted on the wiring board, at least one of the integrated circuit chips is mounted on a first surface of the wiring board as a flip chip with a sealing resin, and a bare plate is mounted with a resin directly opposite the integrated circuit chip on a second surface of the wiring board opposite the first surface.
The invention inhibits the warp of the LSI as described above. Furthermore, the structure where the effect of outgoing radiation is high can be acquired by providing a uniformly thin thermal conduction member on a face where a heat sink and LSI are bonded. Also, in the structure, arranging the chip capacitor on the wiring board between the plate on the second surface and the wiring board can inhibit the rapid variation in voltage of the LSI, and high-speed operation can be realized.